In 1943, the heart of the German air defense system was the early-warning and flak-predictor radar systems. It had proved to be a good system, as was the British radar chain that had been set up since 1935. The Germans knew, however, that strips of metal foil could blind the radar sets and they were reluctant to use this against the enemy, lest the enemy employ the same strategy against them. Despite their hesitation, by mid-1943 the British had decided to put into play this strategy of using foil strips, or chaff, to blind the German radars. The system was called Window, and it would be used for the first time on July 24th, 1943, in a bombing raid over Hamburg, in what would become a devastating blow to this major German city. This would be Operation Gomorrah.